1. Field of the Invention
The present invention relates generally to chemical mechanical polishing of substrates, more particularly to apparatus for, and methods of, chemically mechanically polishing semiconductor substrates and, even more specifically to a substrate carrier and the method of using the carrier in a chemical mechanical polishing apparatus.
2. Background of the Art
In certain technologies, such as integrated circuit fabrication, optical device manufacture and the like, it is often crucial to the fabrication processes involved that the workpiece from which the integrated circuit, optical, or other device is to be formed have a substantially planar front side and, for certain applications, have both a planar front side and back side.
One process for providing such a planar surface is to scour the surface of the substrate with a conformable polishing pad, commonly referred to as "mechanical polishing." When a chemical slurry is used in conjunction with the pad, the combination of slurry and pad generally provides a higher material removal rate than is possible with mere mechanical polishing. This combined chemical and mechanical polishing, commonly referred to as "CMP," is considered an improvement over mere mechanical polishing processes for planarizing or polishing substrates. The CMP technique is common for manufacture of semiconductor wafers used for the fabrication of integrated circuit die.
One recurring problem with CMP processing is the tendency of the process to differentially polish the surface of the substrate and thereby create localized over-polished and under-polished areas across the substrate surface. Where the substrate is to be further processed, such as by photolithographic etching to create integrated circuit structures, thickness variation in the planarized layer makes it extremely difficult to meet the fine resolution tolerances required to provide a high yield of functional die on a wafer.
In typical CMP apparatus, the substrate is received in a substrate carrier mechanism which positions the surface of the substrate to be polished on the pad, and which also provides a bias force between the surface of the substrate and the polishing pad. The carrier mechanism typically includes a recess within which the substrate must be retained for polishing, and within which the substrate should be retained when the carrier is lifted from the polishing pad where proper removal of the substrate from the carrier can be affected by the CMP machine operator.
A variety of techniques have been used to hold the substrate in the carrier. For example, a soft, resilient pad can be placed between a planar substrate mounting plate on the base of the carrier and the substrate, with the substrate held against the resilient pad by surface tension created by compressing the resilient pad with the substrate. In other prior art techniques, a polymer sheet or a wax mound has been used to hold the wafer to a planar substrate mounting plate. These solutions have been found to be less than desirable in resolving substrate handling difficulties in that the combination of the mounting plate and the conformable material may not be as flat as the desired flatness of the substrate and thus the carrier may differentially load the backside of the substrate. Such differential loading would cause localized high polishing pressure regions between the substrate and the pad, which will cause the formation of localized overpolished regions on the polished substrate.
An additional method of holding the substrate to the carrier is shown in U.S. Pat. No. 5,095,661, Gill wherein a vacuum is applied to the backside of the resilient pad against which the substrate is positioned, through one or more ports connected to a vacuum source such as a pump, to provide a releasable chucking means. Typically, the resilient pad is substantially porous, or through holes are also provided in the resilient pad between the carrier plate and the substrate, to create sufficient communication between the vacuum and the substrate to cause suction against the substrate back side to adhere it to the carrier as the carrier is lifted away from the pad. However, this configuration has been found to suck slurry up from the pad and into the vacuum ports and thereby contaminate the carrier mechanism.
Therefore, there is a need for a carrier head for CMP apparatus with improved substrate loading, retaining and unloading capability.